I don't know what you mean by "ever seriously considered for use" — they were developed under deadly serious pretenses, with the belief that they would probably be used. If you are asking, were there situations where commanders ever really wanted to use them — yes, for sure. The most famous close-call in this respect is the Arkhipov incident, in which, during the Cuban Missile Crisis, a Soviet submarine almost used a tactical nuclear torpedo against a ship that was harassing it. Furthermore, the Soviets and Cubans during the Cuban Missile Crisis were quite ready to use tactical nuclear weapons against an invading American force, had one arrived on their beaches.

"Superior choice" depends on a lot of parameters and judgments, not usually limited to the tactical constraints of the battlefield. For example, if one wanted to destroy a built-in fortification in Vietnam or Korea, would a tactical nuke be more capable of effectively guaranteeing that destruction than conventional munitions? Undoubtedly! But would the long-term consequences of that be worth the short-term tactical advantage? This is where nuclear states have consistently (thankfully) concluded "no" (so far, anyway).

What kinds of long-term consequences are they talking about? One is escalation, which is implicit in the last part of your question: that you using nukes might lead another state to feel that nukes were necessary in retaliation. This would especially be the case if you were actively engaged with another state with nuclear weapons, and this was the expectation for any kind of escalating conflict in Western Europe, for example. And one could imagine that it might escalate even with a non-nuclear state who had nuclear patrons, like Vietnam.

But this was not the only possible reason not to use them, however. Any use of nuclear weapons would come with significant negative world opinion, and as much as we might like to imagine that states don't care about world opinion, it is clear that they actually do, especially for wars in which they significantly rely upon the cooperation of allies to wage. The United States could not prosecute its aims in Vietnam, for example, without the good favor of a number of nations — the French, the Japanese, etc. It did worry about overly alienating them, and especially for the Japanese, the use of nuclear weapons was a major taboo.

More generally, various US analysts have also concluded that the major benefactor of the non-use of nuclear weapons is the United States itself. To weaken the norm of non-use would invite other states to also use tactical nuclear weapons towards their aims, states for whom nuclear weapons might provide a much larger increase over their conventional capabilities than they do for the United States. The United States prefers a military style that is particular vulnerable to tactical nuclear weapons (hardened, centralized bases, for example), as well. This was a major internal argument against the use of nukes in Vietnam — that overall, nuclear weapons would likely not make much of a difference against the Vietnamese, but they could make a huge difference against the United States, if their use became normalized.

Anyway — there is much that one can say about these things, but this is an overview. The link about specifically talks about the situation in Vietnam in some depth. Vietnam was one of the last times in which the possible use of tactical nuclear weapons was floated by the United States, and is instructive along a number of lines. We do not have a comparable account of the Soviet Union's war in Afghanistan that I know of — I suspect they contemplated such things there as well.